JPH0331396Y2 - - Google Patents

Description

[Detailed explanation of the idea]

(Field of Industrial Application) The present invention relates to a catalyst for purifying exhaust gas emitted from internal combustion engines such as automobiles. (Prior art) Various catalysts have been proposed for exhaust gas purification. For example, an activated alumina layer is formed on an inorganic carrier base material such as cordierite, and rhodium (Rh) and palladium (Pd) are added to this alumina layer. ), platinum (Pt), and other catalyst metals are known. Among the above catalyst metals, Pt is most often used in combination with rhodium because it has excellent properties such as activity and durability, but it has the disadvantage of being expensive. Therefore, in recent years, attempts have been made to use Pd/Rh-based catalysts as three-way catalysts in which palladium, which is cheaper than platinum and exhibits high activity in oxidation reactions, is used instead of platinum. However, conventional catalysts of this type cannot overcome the disadvantages of Pd, such as being easily sintered and easily poisoned in a reducing atmosphere, and have had the problem of lower activity than Pt/Rh catalysts. (Purpose of the invention) The present invention is intended to solve the problems of the prior art described above, and its purpose is to overcome the drawbacks of Pd and to develop an inexpensive Pd catalyst that has an activity equal to or higher than that of Pt/Rh catalysts. /Rh catalyst. (Structure of the invention) In other words, the exhaust gas purification catalyst of the present invention has activated alumina layers laminated in two layers, upper and lower, on the surface of an inorganic carrier base material made of cordierite, etc., with palladium in the lower layer and palladium and rhodium in the upper layer. is supported such that the weight ratio of palladium in the lower layer to palladium in the upper layer is 7/3 to 9/1, and the weight ratio of palladium to rhodium in the upper layer is 3/1 to 1/1. . Palladium is easily sintered in a reducing atmosphere, and when such sintering occurs, the number of active sites decreases, resulting in a decrease in activity. They are also easily poisoned by harmful substances contained in exhaust gas, such as lead and phosphorus. Therefore, in order to solve the above-mentioned drawbacks, palladium is supported by dividing the active alumina layers stacked into two layers, with more palladium being supported in the lower layer (inner layer) and less palladium in the upper layer (outer layer). The palladium supported on the upper layer is exposed to higher temperatures during exhaust gas purification than the lower layer, and is poisoned, resulting in a decrease in catalytic activity, but on the other hand, this protects the palladium on the lower layer. Therefore, the catalyst performance can be improved as a whole compared to when the catalyst is uniformly supported. In this case, it is important to optimally select the distribution ratio between the upper layer and the lower layer. Palladium supported on the upper layer is Pd (top), palladium supported on the lower layer is Pd
(Bottom): Pd (bottom) / Pd (top) (weight ratio)
is 7/3 to 9/1. Palladium is usually supported in an amount of 0.5 to 3 g per catalyst carrier. Rhodium is supported in the upper layer because it has excellent durability and toxicity resistance at high temperatures. Like platinum, rhodium is expensive, so it is better to use as little as possible. Further, the ratio of use to palladium (total) is also preferably as small as possible. Sufficient catalytic performance can be obtained if the amount used is such that the weight ratio of palladium to rhodium in the upper layer is 3/1 to 1/1 [the weight ratio of palladium (total) to rhodium is 10/1]. Therefore, rhodium is usually
Carrying 0.05g to 0.3g. Two activated alumina layers are usually sufficient, but
If necessary, it may be further divided into multiple layers.
As the type of activated alumina, those normally used for this type of purpose, such as θ- or γ-alumina with an average particle size of about 10 μm, can be used. Conventional methods can be used for forming the activated alumina layer and supporting the catalyst metal. (Example) An example of the present invention will be described below along with a reference example and a comparative example. Note that the present invention is not limited to the following examples. Reference example 1: Commercially available alumina sol with alumina content of 10% by weight
70 parts by weight of activated alumina powder, 100 parts by weight of activated alumina powder, and 20 parts by weight of water were thoroughly mixed and stirred to obtain an alumina slurry. The cordierite monolith carrier base material was immersed in this alumina slurry for 1 minute and pulled up, the slurry remaining in the cell was blown off with an air stream, dried at 200°C for 1 hour, and then fired at 700°C for 2 hours to form an activated alumina layer. A was formed. Layer thickness is approximately 40μm
It was hot. Next, it was immersed in an aqueous palladium chloride solution for 1 hour, and then immersed in an aqueous sodium borohydride solution for reduction to support palladium, and then washed with hot water at 80°C to wash away the sodium. Further, an alumina layer B containing no noble metal was formed on the alumina layer A by the same operation using an alumina slurry having the same composition as above. Next, by the same operation as above, the alumina layer B was impregnated with a palladium chloride aqueous solution having a lower concentration than the above, reduced, and washed with hot water. Next, after soaking in rhodium chloride aqueous solution for 1 hour,
After drying for hours, catalyst 1 of Reference Example 1 was obtained. Reference Example 2 and Examples 1 to 3 Catalyst 2 of Reference Example 2 was prepared in the same manner as in Reference Example 1, except that only the amount of palladium supported on alumina layer A (lower layer) and alumina layer B (upper layer) was changed.
And catalysts 1 to 3 of Examples 1 to 3 were prepared. FIG. 1 shows a perspective view of an exhaust gas purifying catalyst 1 of the present invention. FIG. 2 is a partially enlarged sectional view of the above catalyst, in which 2 represents the cordierite carrier base material, 3 represents the alumina layer A, and 4 represents the alumina layer B. Comparative Example 1: Using an alumina slurry having the same composition as in Reference Example 1, a cordierite monolithic carrier base material was immersed under the same conditions to form an alumina layer. At this time, in order to make the thickness of the alumina layer the same as in Reference Example 1, the above operation was repeated twice. Pd was supported on this using an aqueous palladium chloride solution in the same manner as in Reference Example 1, and then Rh was supported using an aqueous rhodium chloride solution to obtain a catalyst of Comparative Example 1. Comparative Example 2: A catalyst of Comparative Example 2 was prepared in the same manner as in Comparative Example 1, but using a dinitrodiaminoplatinum aqueous solution instead of the palladium chloride aqueous solution. Table 1 shows the amount of catalyst metal supported in the catalysts of Reference Examples 1 and 2, Examples 1 and 3, and Comparative Examples 1 and 2.

[Table] Catalytic metal was uniformly supported throughout the Mina layer.
Durability Test: The catalysts of Reference Example, Example, and Comparative Example were loaded into a catalytic converter, which was connected to the exhaust system of a 6-cylinder 2.8 engine, and a durability test was conducted under the following conditions. <Test conditions> Air-fuel ratio (A/F) 14.6 Space velocity 60,000 hr ^-1 Catalyst bed temperature 700°C Test time 300 hr Purification performance evaluation test: After the durability test, the catalyst was purified under the following conditions using model gas. Performance was evaluated. <Test conditions> Model gas composition (volume %) CO; 0.8%, NOx;
0.2% C ₃ H ₆ ; 0.08%, O ₂ ; 0.8% H ₂ ; 0.17%, H ₂ O; 10% CO ₂ ; 10%, balance N ₂ Space velocity 90,000 hr ^-1 Catalyst inlet gas temperature 300°C Results are shown in Table 2.

[Table] Comparing Reference Examples 1 to 2 and Examples 1 to 3, the initial activity is determined by the Pd concentration on the surface, that is, the alumina layer B.
The higher the Pd concentration is, the better; conversely, the activity after durability is better when the Pd concentration inside, that is, in the alumina layer A, is higher to some extent, and the catalysts of Examples 1 to 3 have almost the same initial activity and activity after durability. It can be seen that the activity is more stable than the catalysts of Reference Examples 1 and 2. Furthermore, when comparing the catalysts of Reference Example 1 and Comparative Example 1, it can be seen that the catalyst of Reference Example 1 has better activity both at the initial stage and after durability, and that the abundance ratio of Pd and Rh is appropriate. Furthermore, the catalysts of Examples 1 to 3 all had higher activity than the catalyst of Comparative Example 1 both at the initial stage and after durability.
Almost equivalent to the Pt/Rh-based catalyst of Comparative Example 2,
This means that even if the same amount of Pd and Rh is used, rhodium, which has high properties for reduction reactions, is supported only in the upper layer, and has high activity for oxidation reactions. This shows that more palladium supported in the lower layer gives more favorable results. The optimal distribution ratio of Pd to alumina layer A and alumina layer B is 7/3 to 9/1 considering the overall characteristics, and the ratio of Pd to Rh in alumina layer B is 3/1 to 1/1. is optimal. (Effect of the invention) As mentioned above, the exhaust gas purification catalyst of the present invention uses cheaper palladium instead of expensive platinum, and in combination with rhodium, the upper and lower layers, especially the lower activated alumina layer, have a large amount of palladium. By optimally distributing palladium and selecting the ratio of palladium and rhodium in the upper layer, we can overcome the disadvantages of palladium such as being easily sintered and easily poisoned.
In terms of overall characteristics, it has catalytic performance equivalent to or better than conventional platinum/rhodium-based catalysts, and is highly effective in reducing catalyst manufacturing costs.

[Brief explanation of the drawing]

FIG. 1 is a perspective view of the exhaust gas purifying catalyst of the present invention, and FIG. 2 is a partially enlarged sectional view of the catalyst of FIG. 1. In the figure, 1... Exhaust gas purification catalyst, 2... Cordierite carrier base material, 3... Alumina layer A, 4
...Alumina layer B.

Claims (1)

[Scope of utility model registration request] Activated alumina layers are laminated in two layers, upper and lower, on the surface of an inorganic carrier base material made of cordierite, etc., with palladium in the lower layer, palladium and rhodium in the upper layer, and a weight ratio of palladium in the lower layer to palladium in the upper layer is 7/3. An exhaust gas purifying catalyst characterized in that the weight ratio of palladium and rhodium in the upper layer is 3/1 to 1/1.